This invention relates to a method and apparatus for heating thermoplastic blanks or preforms preparatory to the blow molding of hollow articles therefrom, for example, bottles.
In the process of manufacturing hollow articles from thermoplastics, the first operation is that of forming preforms having a neck with the same final shape and size of the neck of the hollow article one wishes to make. These blanks are then heated to a temperature corresponding to the state of thermoelasticity of the material of which they are composed, following which they are placed in a mold in which the final shape of the finished product is obtained through blowing.
The most frequently used means for heating the preforms consists of a tunnel provided with a longitudinal slot in its lower surface. At least one of the lateral walls of the tunnel is equipped with infra-red radiators. The preforms, which protrude vertically into the tunnel through the longitudinal slot, are placed neck down on rotating supports that move along the longitudinal axis of the tunnel.
Also known is a tunnel of the above type in which a fan blows cool air onto the connections and rear surfaces of the infra-red radiator reflectors to cool them and thereby prolong their life. This air, which becomes heated in the process of cooling the connections and reflectors, then enters the tunnel through openings provided in or between the reflectors and homogenizes the temperature of the tunnel before escaping to the outside through the longitudinal slot in the lower surface. Such a tunnel has the drawback of exhausting hot air in the vicinity of the necks of the preforms. This has the effect of heating and possibly deforming them.
Also known is a tunnel open at the bottom and heated by infra-red radiators, in which the upper surface is provided with breather ports that may be closed using flap valves controlled by at least one thermostatic sensor. In this tunnel the heating of the preforms can be regulated not only by varying the power supplied to the radiators, but also by opening the breather valves controlled by the thermostatic sensors. If the temperature rises the valves open, increasing the ventilation in the tunnel and producing a regulating effect. By natural convection, air flows from the bottom to the top of the tunnel and, after being heated, is exhausted through the open valves. This tunnel has the drawback of cooling the necks of the preforms only when the flap valves are open.